David Zhu

alluxio-bot, merge this please

high level comment, why do we only force sync when the length has changed to be greater? why do we not also sync when the length changed shorter? is it...

@yyongycy high level comment, could you add some user documentation?

a high level question, does this invalidation take effect even if metadata sync is turned off. (ie. meta sync interval = -1)

alluxio-bot, merge this please

@adol001 thanks for the fix. have you tested this change with a deep directory structure and compare the two options to see memory usage and performance difference? would be curious...

alluxio-bot, merge this please

@StephenRi thanks for improving our docs. Do you mind adding a similar section to the english docs as well? and please target the master branch with your doc change. We...

@Sucran thanks for the note. Please make a similar PR targeting master branch. We can only take contributions targetting master branch.

@beinan could you take a look at this? since you have some experience with M1?